This invention relates generally to electronic circuits, and more particularly, to an adjustable current limiting/sensing circuit and method of using same.
In designing semiconductor power integrated circuits (ICs), it is often prudent to include limiting/sensing circuitry to monitor and, if necessary, control the IC""s output current or current flowing through the circuit. For example, current limiting/sensing circuitry may be provided to monitor and control the output current of high or low power IC switches or amplifiers. Also, current limiting/sensing circuitry may be used in an IC for fault protection, such as, for example, in detecting internal or external short circuits. In such an application, whenever an excessive amount of current is detected flowing either in the IC or through its output structure, the current limiting/sensing circuit may be designed to xe2x80x9ctripxe2x80x9d and turn the IC, or if desired, any associated circuitry xe2x80x9coffxe2x80x9d.
Using a typical design for a current limiting/sensing circuit in an IC, a portion of the output current to be monitored is passed through a resistor. A voltage drop is developed across the resistor, which is proportional to the output current. Consequently, as the IC""s output current is increased, the current through the resistor is increased, which increases the voltage drop across the resistor. Conversely, as the output current is decreased, the voltage drop across the resistor is accordingly decreased. Therefore, by monitoring the voltage drop across the resistor, the IC""s output current may be monitored and thereby controlled, if so desired.
FIG. 1 shows a typical semiconductor power IC design that includes a current sensing circuit having a FET arranged in combination with a diffusion or poly-silicon resistor to form a voltage divider. With an appropriate signal applied to the GATE IN input lead, IC 10 will conduct or be turned xe2x80x9conxe2x80x9d. Current I1 passes through sensing transistor MD1 and resistor R1, while current I2 passes through output transistor MDout. Current I1 is proportional to current I2, as defined by the width/length (w/l) ratio of MD1 to the w/l ratio of MDout. In general, current I1 is much smaller than current I2 (e.g., 500:1 or greater). The voltage developed at node 4, with respect to xe2x80x9cgroundxe2x80x9d, varies proportionally with the output current Iout. However, a problem exists in manufacturing circuits such as IC 10, because the transistors and resistors are fabricated by different processes. For example, the transistors may be fabricated as double-diffused MOS (DMOS) devices, while the resistors may be fabricated by a separate, diffusion process. Consequently, these process variations between components typically cause significant deviations in the trip/sensing levels experienced from IC to IC. Furthermore, since the performance characteristics of the components in each IC are somewhat process dependent, thermal gradients often occur between the components that contribute to the trip/sensing level errors. Therefore, accurate (i.e., reduced error) current limiting/sensing circuits are difficult to fabricate and implement in power and similar ICs.
Additionally, in order to change a trip/sensing level in a typical IC, a user must change one or more of the components"" values. For example, due to the above-described process variations, if a user requires a specific trip/sensing voltage Vlimitxe2x80x94out to represent a specific output current Iout in IC 10, either the IC would have to be custom made to provide the specific values (thereby increasing fabrication costs), or the user would have to be afforded the option of changing the trip/sensing level for each IC. However, in order for a user to change the trip/sensing level of IC 10 in FIG. 1, either the value of resistor R1 or the gain ratio of transistors MD1 and MDout would have to be changed. Consequently, to facilitate such changes, a user may be given the option of changing the metal leads mask of the IC, or activating poly-fuses or zener zap circuits incorporated in the IC during fabrication. Such limited flexibility increases the complexity and fabrication costs of the IC. Therefore, given the errors introduced into current limiting/sensing circuits due to process variations and thermal gradients, and the relative inflexibility of their design, accurate (i.e, reduced errors) limiting/sensing circuits are difficult to fabricate and implement in power and similar ICs.
Accordingly, a need exists in the integrated circuit manufacturing industry for an accurate, adjustable current limiting/sensing circuit. In accordance with the present invention, a method and circuitry are provided for accurate and adjustable current limiting/sensing in a power or similar IC. In particular, a current limiting/sensing circuit and method of use may be provided that substitutes a transistor in place of a resistor or similarly xe2x80x9cnonadjustablexe2x80x9d component. Consequently, all of the critical components in the IC may be fabricated by one process and integrated in one power structure.
An important technical advantage of the present invention is that errors due to process variations and thermal gradients may be minimized using identical components fabricated by one process and integrated into one power output structure. Another important technical advantage of the present invention is that the current trip/sensing level of the IC may be adjusted by applying an appropriate gate voltage to the substituted transistor. Consequently, the adjustable, current limiting/sensing circuit of the present invention is significantly more accurate and flexible than existing current limiting/sensing circuits.